Dl. Davidson et Rc. Mcclung, LOCAL CONSTRAINT NEAR FATIGUE CRACKS IN ALLOYS AND PARTICULATE COMPOSITES, International journal of fracture, 84(1), 1997, pp. 81-98
An experimental method with high spatial resolution has been developed
for determining material constraint on the surface of specimens. The
method uses strains computed from stereoimaging measurements of displa
cements. A derivation of constraint in terms of strain is given and co
mpared to the usual stress definition. Constraint was found to be high
near fatigue crack tips growing at low Delta K. Monotonic loading of
a crack tip that formed shear bands relaxed constraint in the regions
of high deformation, as expected. Constraint levels in particulate rei
nforced metal matrix composites depend on proximity of the crack tip t
o particles. In general, particulate composites show higher levels of
constraint than the matrix alloy for the same K level, which partially
explains why the composites have lower ductility and fracture toughne
ss.